The present invention relates to an optical device for achieving channel equalizing amplification of the power level of optical wavelength channels.
Several different methods of increasing the capacity of existing optical networks are known. One way is to use so-called wavelength multiplexing (WDM) techniques to enhance the extent to which available bandwidths can be utilised on an optical fibre in the optical Network. In an optical network, the wavelength can also be used as an information address, that is to say the information can be multiplexed on a number of channels which can then be processed individually in the network. This can result in different channels being subjected to losses of different magnitudes, among other things because the various channels are attenuated to differing degrees in filter structures and in switch structures, because said channels take paths of different lengths through the network, or because said channels are amplified to different extents in optical amplifiers. This imbalance can impair the quality of the transmitted information, since a channel that has a low power level can be easily disturbed by a channel that has a high power level, which is normally referred to as cross-talk.
One known device that achieves channel equalization of optical channels is an equalizer based on multiplexing/demultiplexing elements and variable optical attenuators. The problem with this solution is that the optical channels are equalized by attenuating high power levels. Another problem with this solution is that performance impairing interference powers can occur.
Any one of a number of known methods can be used to increase the capacity of an optical transmission system. In the case of wavelength multiplexing for instance, transmission channels are multiplexed and demultiplexed on different carrier wave lengths to and from an information stream respectively, This multiplexing and demultiplexing requires the presence of optical wavelength selective devices. Different transmission channels are subjected to losses of different high magnitudes, among other things because the various transmission channels are attenuated to different extents in filter and switch structures, because said channels pass through the network in paths of mutually different lengths, or because the channels are amplified to different extents in optical amplifiers.
One problem with known channel equalizers is that they attenuate the highest channel-power levels, which is a waste of power and can considerably impair performance.
Another problem with known channel equalizers is that they are sensitive to interference powers, which can result in further impairment of performance.
The present invention addresses these problems with an optical channel equalizer that includes at least one direction-dependent router that has Q-number of ports, where Qxe2x89xa73, one WDM (de)multiplexer that has N-number of channels, where Nxe2x89xa72, N-number of amplifying waveguides, where each amplifying waveguide includes at least one fibre amplifier and at least one Bragg grating, at least N number of variable optical pump laser attenuators, at least one optical splitter, and at least one pump laser per optical splitter. At least one of the ports On the direction-dependent router is disposed on a first side of the N-channel WDM (de)multiplexer. At least one fibre amplifier is disposed between a Bragg grating and the WDM (de)multiplexer. At least one variable optical pump laser attenuator is disposed between each last Bragg grating and a first side of said optical splitter. The pump laser is disposed on the other side of the optical splitter.
In a preferred embodiment of the inventive channel equalizer, the Q-port direction-dependent router is a Q-port optical circulator.
The N-channel WDM (de)multiplexer may, for instance, be an AWG (Arrayed Waveguide Grating) or an MMIMZI (Multi Mode Interference Mach-Zehnder Interferometer).
In another embodiment of the inventive channel equalizer, said equalizer includes at least one Q-port direction-dependent router, where Qxe2x89xa73, an N-channel WDM (de)multiplexer, where Nxe2x89xa72, N-number of amplifying waveguides, where each amplifying waveguide includes at least one fibre amplifier and at least one Bragg grating, and at least one pump laser per amplifying waveguide. At least one of the ports on the direction-dependent router is disposed on a first side of said N-channel WDM (de)multiplexer. At least one fibre amplifier is disposed between a Bragg grating and the WDM (de)multiplexer. The pump laser is disposed at the end of each amplifying waveguide.
In one method according to the present invention for equalizing the power level of optical channels, optical wavelength channels are first transmitted into a first port on a Q-port direction-dependent router. The wavelength channels are then transmitted out through a second port on said router, which is disposed on a first side of an N-channel WDM (de)multiplexer. The wavelength channels are then transmitted through said WDM (de)multiplexer. Different wavelength channels are then transmitted through different amplifying waveguides, For each amplifying waveguide, a wavelength channel passes at least one optical amplifier before being reflected by a Bragg grating. Laser light is pumped into each amplifying waveguide in a direction towards the WDM (de)multiplexer. The reflected optical wavelength channels are transmitted through said WDM (de)multiplexer. These reflected wavelength channels are transmitted in through said second port on said Q-port direction-dependent router, so as to be finally transmitted out through a third port on said router.
The object of the present invention is to provide an arrangement for channel equalizing amplification of the power level of WDM channels, with which channels that have a low power level are amplified to a greater extent that channels that have high power levels. One advantage afforded by the present invention is that dispersion compensation can be made for each channel when the period in the grating structures varies.
Another advantage afforded by the invention is that its performance in other respects can be improved relative to known techniques, for instance with respect to cross-talk and the like.
Another advantage afforded by the present invention is that a high level of reliability can be achieved by using a solution in which at least two pump lasers pump laser light to all fibre amplifiers disposed in the amplifying waveguides, and in which at least one of these pump lasers can be driven harder when replacing a malfunctioning pump laser.
The present invention will now be described in more detail with reference to preferred exemplifying embodiments thereof and also with reference to the accompanying drawings.